Australian Water Management Review Vol. 1 2014 | Page 44

Exposing
Sydney’s hidden
marine world

S

ydney Water collects and treats about
1.3 billion litres of wastewater each day.
This is transported through a network of
24,000 kilometres of wastewater pipes
and 680 pumping stations to 14 water
recycling plants and 16 treatment plants across
Sydney, the Illawarra and the Blue Mountains.
Approximately 80% of Sydney’s sewage is treated
at the North Head, Bondi and Malabar Wastewater

Treatment Plants and discharged through three deep
ocean outfalls located between two and five kilometres
offshore and between 40 and 80 metres deep.
It wasn’t always this way. Up until the early 1990s
wastewater was disposed of through cliff face
outfalls which would lead to visible effluent plumes
off Sydney’s headlands.
To reduce the environmental impact and protect
Sydney’s beaches a project was devised to design a
system to safely dilute and disperse into the ocean a
large portion of Sydney’s treated waste.
The Deep Ocean Outfall project was adopted by the
then Sydney Water board as the preferred solution
for stopping beach pollution in 1980. Constructed
started within 1 year with the main tunnelling
starting at North Head, Bondi and Malabar
Wastewater Treatment Plants in 1984.
The total cost to investigate, design, construct and
commission the three DOOs was $310 million.
To meet the completion deadline of 1991, work
continued simultaneously on all three ocean
outfalls, often 24 hours a day.

At the time, the hydraulic design of the Sydney
Outfalls represented the leading edge of
development for high dilution, self-cleansing and
essentially maintenance free outfall facilities. The
design has since been adopted for other major
overseas outfalls, including Boston, USA.
Each of the three deep water submarine outfalls
essentially consisted of a main tunnel constructed
from a point beneath the treatment plants and
extending for a distance up to 3 kilometres offshore.
Treated wastewater is discharged into the ocean at
depths ranging from 40 to 80 metres. At North Head
there are 37 vertical risers, at Bondi there are 27
vertical risers and at Malabar there are 29 vertical
risers. Each of the fibreglass risers is encased in
concrete and is connected to a fibreglass diffuser
head, while large steel caissons of up to 3 metres
in diameter protect the 8-port diffuser heads from
ships anchors.

All three sites were completed on time, with Malabar
starting operation in 1989, then North Head and
Bondi in August 1991.
Construction involved a unique combination of
tunnelling and offshore engineering technologies,
the latter being adapted from similar offshore oil and
gas industry technology.
38 | Australian water man age m e nt re v ie w

The diffusers, parallel to the ocean currents, release
effluent into deep water which ensures adequate
dilution. Salt water, sunlight, and wave action
assist in breaking down and disinfecting the treated
wastewater. Each outfall is designed to uniformly
discharge the effluent into the ocean along a 500800 metre diffuser zone, 2.2-2.7km offshore.
Depending on the diffuser field the wastewater is
diluted at up to 300-500 times. This means each
molecule of wastewater is diluted up to 500 times
by sea water.